journalctl — Query the systemd journal
journalctl  [OPTIONS...] [MATCHES...]
journalctl may be used to query the contents of the systemd(1) journal as written by systemd-journald.service(8).
If called without parameters, it will show the full contents of the journal, starting with the oldest entry collected.
If one or more match arguments are passed, the output is
      filtered accordingly. A match is in the format
      "FIELD=VALUE",
      e.g. "_SYSTEMD_UNIT=httpd.service", referring
      to the components of a structured journal entry. See
      systemd.journal-fields(7)
      for a list of well-known fields. If multiple matches are
      specified matching different fields, the log entries are
      filtered by both, i.e. the resulting output will show only
      entries matching all the specified matches of this kind. If two
      matches apply to the same field, then they are automatically
      matched as alternatives, i.e. the resulting output will show
      entries matching any of the specified matches for the same
      field. Finally, the character "+" may appear
      as a separate word between other terms on the command line. This
      causes all matches before and after to be combined in a
      disjunction (i.e. logical OR).
It is also possible to filter the entries by specifying an
      absolute file path as an argument. The file path may be a file or
      a symbolic link and the file must exist at the time of the query. If a
      file path refers to an executable binary, an "_EXE="
      match for the canonicalized binary path is added to the query. If a
      file path refers to an executable script, a "_COMM="
      match for the script name is added to the query. If a file path
      refers to a device node, "_KERNEL_DEVICE=" matches for
      the kernel name of the device and for each of its ancestor devices is
      added to the query. Symbolic links are dereferenced, kernel names are
      synthesized, and parent devices are identified from the environment at
      the time of the query. In general, a device node is the best proxy for
      an actual device, as log entries do not usually contain fields that
      identify an actual device. For the resulting log entries to be correct
      for the actual device, the relevant parts of the environment at the time
      the entry was logged, in particular the actual device corresponding to
      the device node, must have been the same as those at the time of the
      query. Because device nodes generally change their corresponding devices
      across reboots, specifying a device node path causes the resulting
      entries to be restricted to those from the current boot.
Additional constraints may be added using options
      --boot, --unit=, etc., to
      further limit what entries will be shown (logical AND).
Output is interleaved from all accessible journal files, whether they are rotated or currently
      being written, and regardless of whether they belong to the system itself or are accessible user
      journals. The --header option can be used to identify which files
      are being shown.
The set of journal files which will be used can be
      modified using the --user,
      --system, --directory, and
      --file options, see below.
All users are granted access to their private per-user
      journals. However, by default, only root and users who are
      members of a few special groups are granted access to the system
      journal and the journals of other users. Members of the groups
      "systemd-journal", "adm", and
      "wheel" can read all journal files. Note
      that the two latter groups traditionally have additional
      privileges specified by the distribution. Members of the
      "wheel" group can often perform administrative
      tasks.
The output is paged through less by
      default, and long lines are "truncated" to screen width. The
      hidden part can be viewed by using the left-arrow and
      right-arrow keys. Paging can be disabled; see the
      --no-pager option and the "Environment" section
      below.
When outputting to a tty, lines are colored according to priority: lines of level ERROR and higher are colored red; lines of level NOTICE and higher are highlighted; lines of level DEBUG are colored lighter grey; other lines are displayed normally.
The following options are understood:
--no-full, --full, -l¶Ellipsize fields when they do not fit in available columns. The default is to show full fields, allowing them to wrap or be truncated by the pager, if one is used.
The old options
          -l/--full are not useful
          anymore, except to undo --no-full.
-a, --all¶Show all fields in full, even if they include unprintable characters or are very long. By default, fields with unprintable characters are abbreviated as "blob data". (Note that the pager may escape unprintable characters again.)
-f, --follow¶Show only the most recent journal entries, and continuously print new entries as they are appended to the journal.
-e, --pager-end¶Immediately jump to the end of the journal
          inside the implied pager tool. This implies
          -n1000 to guarantee that the pager will not
          buffer logs of unbounded size. This may be overridden with
          an explicit -n with some other numeric
          value, while -nall will disable this cap.
          Note that this option is only supported for the
          less(1)
          pager.
-n, --lines=¶Show the most recent journal events and
          limit the number of events shown. If
          --follow is used, this option is
          implied. The argument is a positive integer or
          "all" to disable line limiting. The default
          value is 10 if no argument is given.
--no-tail¶Show all stored output lines, even in follow
          mode. Undoes the effect of --lines=.
          
-r, --reverse¶Reverse output so that the newest entries are displayed first.
-o, --output=¶Controls the formatting of the journal entries that are shown. Takes one of the following options:
short
              ¶is the default and generates an output that is mostly identical to the formatting of classic syslog files, showing one line per journal entry.
short-full
              ¶is very similar, but shows timestamps in the format the --since= and
                --until= options accept. Unlike the timestamp information shown in
                short output mode this mode includes weekday, year and timezone information in the
                output, and is locale-independent.
short-iso
              ¶is very similar, but shows ISO 8601 wallclock timestamps.
short-iso-precise
              ¶as for short-iso but includes full
                microsecond precision.
short-precise
              ¶is very similar, but shows classic syslog timestamps with full microsecond precision.
short-monotonic
              ¶is very similar, but shows monotonic timestamps instead of wallclock timestamps.
short-unix
              ¶is very similar, but shows seconds passed since January 1st 1970 UTC instead of wallclock timestamps ("UNIX time"). The time is shown with microsecond accuracy.
verbose
              ¶shows the full-structured entry items with all fields.
export
              ¶serializes the journal into a binary (but mostly text-based) stream suitable for backups and network transfer (see Journal Export Format for more information). To import the binary stream back into native journald format use systemd-journal-remote(8).
json
              ¶formats entries as JSON objects, separated by newline characters (see Journal JSON Format for more information). Field values are generally encoded as JSON strings, with three exceptions:
Fields larger than 4096 bytes are encoded as null values. (This
                  may be turned off by passing --all, but be aware that this may allocate overly long
                  JSON objects.) 
Journal entries permit non-unique fields within the same log entry. JSON does not allow non-unique fields within objects. Due to this, if a non-unique field is encountered a JSON array is used as field value, listing all field values as elements.
Fields containing non-printable or non-UTF8 bytes are encoded as arrays containing the raw bytes individually formatted as unsigned numbers.
Note that this encoding is reversible (with the exception of the size limit).
json-pretty
              ¶formats entries as JSON data structures, but formats them in multiple lines in order to make them more readable by humans.
json-sse
              ¶formats entries as JSON data structures, but wraps them in a format suitable for Server-Sent Events.
json-seq
              ¶formats entries as JSON data structures, but prefixes them with an ASCII Record Separator
                character (0x1E) and suffixes them with an ASCII Line Feed character (0x0A), in accordance with JavaScript Object Notation (JSON) Text Sequences 
                ("application/json-seq").
                
cat
              ¶generates a very terse output, only showing the actual message of each journal entry
                with no metadata, not even a timestamp. If combined with the
                --output-fields= option will output the listed fields for each log record,
                instead of the message.
with-unit
              ¶similar to short-full, but prefixes the unit and user unit names instead of the traditional syslog identifier. Useful when using templated instances, as it will include the arguments in the unit names.
--output-fields=¶A comma separated list of the fields which should be included in the output. This has
        an effect only for the output modes which would normally show all fields (verbose,
        export, json, json-pretty,
        json-sse and json-seq), as well as on cat. For the
        former, the "__CURSOR", "__REALTIME_TIMESTAMP",
        "__MONOTONIC_TIMESTAMP", and "_BOOT_ID" fields are always
        printed.
--utc¶Express time in Coordinated Universal Time (UTC).
--no-hostname¶Don't show the hostname field of log messages originating from the local host. This
        switch has an effect only on the short family of output modes (see above).
        
Note: this option does not remove occurrences of the hostname from log entries themselves, so it does not prevent the hostname from being visible in the logs.
-x, --catalog¶Augment log lines with explanation texts from the message catalog. This will add explanatory help texts to log messages in the output where this is available. These short help texts will explain the context of an error or log event, possible solutions, as well as pointers to support forums, developer documentation, and any other relevant manuals. Note that help texts are not available for all messages, but only for selected ones. For more information on the message catalog, please refer to the Message Catalog Developer Documentation.
Note: when attaching journalctl
        output to bug reports, please do not use
        -x.
-q, --quiet¶Suppresses all informational messages (i.e. "-- Journal begins at …", "-- Reboot --"), any warning messages regarding inaccessible system journals when run as a normal user.
-m, --merge¶Show entries interleaved from all available journals, including remote ones.
-b [[ID][±offset]|all], --boot[=[ID][±offset]|all]¶Show messages from a specific boot. This will
        add a match for "_BOOT_ID=".
The argument may be empty, in which case logs for the current boot will be shown.
If the boot ID is omitted, a positive
        offset will look up the boots
        starting from the beginning of the journal, and an
        equal-or-less-than zero offset will
        look up boots starting from the end of the journal. Thus,
        1 means the first boot found in the
        journal in chronological order, 2 the
        second and so on; while -0 is the last
        boot, -1 the boot before last, and so
        on. An empty offset is equivalent
        to specifying -0, except when the current
        boot is not the last boot (e.g. because
        --directory was specified to look at logs
        from a different machine).
If the 32-character ID is
        specified, it may optionally be followed by
        offset which identifies the boot
        relative to the one given by boot
        ID. Negative values mean earlier
        boots and positive values mean later boots. If
        offset is not specified, a value of
        zero is assumed, and the logs for the boot given by
        ID are shown.
The special argument all can be
        used to negate the effect of an earlier use of
        -b.
--list-boots¶Show a tabular list of boot numbers (relative to the current boot), their IDs, and the timestamps of the first and last message pertaining to the boot.
-k, --dmesg¶Show only kernel messages. This implies
        -b and adds the match
        "_TRANSPORT=kernel".
-t, --identifier=SYSLOG_IDENTIFIER¶Show messages for the specified syslog
        identifier
        SYSLOG_IDENTIFIER.
This parameter can be specified multiple times.
-u, --unit=UNIT|PATTERN¶Show messages for the specified systemd unit
        UNIT (such as a service unit), or
        for any of the units matched by
        PATTERN.  If a pattern is
        specified, a list of unit names found in the journal is
        compared with the specified pattern and all that match are
        used. For each unit name, a match is added for messages from
        the unit
        ("_SYSTEMD_UNIT="),
        along with additional matches for messages from systemd and
        messages about coredumps for the specified unit. A match
        is also added for "UNIT_SYSTEMD_SLICE=",
        such that if the provided UNITUNIT is a
        systemd.slice(5)
        unit, all logs of children of the slice will be shown.
       
This parameter can be specified multiple times.
--user-unit=¶Show messages for the specified user session
        unit. This will add a match for messages from the unit
        ("_SYSTEMD_USER_UNIT=" and
        "_UID=") and additional matches for messages
        from session systemd and messages about coredumps for the
        specified unit. A match
        is also added for "_SYSTEMD_USER_SLICE=",
        such that if the provided UNITUNIT is a
        systemd.slice(5)
        unit, all logs of children of the unit will be shown.
This parameter can be specified multiple times.
-p, --priority=¶Filter output by message priorities or
        priority ranges. Takes either a single numeric or textual log
        level (i.e. between 0/"emerg" and
        7/"debug"), or a range of numeric/text log
        levels in the form FROM..TO. The log levels are the usual
        syslog log levels as documented in
        syslog(3),
        i.e. "emerg" (0),
        "alert" (1), "crit" (2),
        "err" (3), "warning" (4),
        "notice" (5), "info" (6),
        "debug" (7). If a single log level is
        specified, all messages with this log level or a lower (hence
        more important) log level are shown. If a range is specified,
        all messages within the range are shown, including both the
        start and the end value of the range. This will add
        "PRIORITY=" matches for the specified
        priorities.
--facility=¶Filter output by syslog facility. Takes a comma-separated list of numbers or facility
        names. The names are the usual syslog facilities as documented in
        syslog(3).
        --facility=help may be used to display a list of known facility names and exit.
        
-g, --grep=¶Filter output to entries where the MESSAGE=
        field matches the specified regular expression. PERL-compatible regular expressions
        are used, see
        pcre2pattern(3)
        for a detailed description of the syntax.
If the pattern is all lowercase, matching is case insensitive.
        Otherwise, matching is case sensitive. This can be overridden with the
        --case-sensitive option, see below.
--case-sensitive[=BOOLEAN]¶Make pattern matching case sensitive or case insensitive.
-c, --cursor=¶Start showing entries from the location in the journal specified by the passed cursor.
--cursor-file=FILE¶If FILE exists and contains a
        cursor, start showing entries after this location.
        Otherwise show entries according to the other given options. At the end,
        write the cursor of the last entry to FILE. Use
        this option to continually read the journal by sequentially calling
        journalctl.
--after-cursor=¶Start showing entries from the location in the
        journal after the location specified by
        the passed cursor.  The cursor is shown when the
        --show-cursor option is used.
--show-cursor¶The cursor is shown after the last entry after two dashes:
-- cursor: s=0639…
The format of the cursor is private and subject to change.
-S, --since=, -U, --until=¶Start showing entries on or newer than the specified date, or on or older than the specified
        date, respectively. Date specifications should be of the format "2012-10-30 18:17:16".  If the
        time part is omitted, "00:00:00" is assumed.  If only the seconds component is omitted,
        ":00" is assumed. If the date component is omitted, the current day is assumed. Alternatively
        the strings "yesterday", "today", "tomorrow" are understood,
        which refer to 00:00:00 of the day before the current day, the current day, or the day after the current day,
        respectively. "now" refers to the current time. Finally, relative times may be specified,
        prefixed with "-" or "+", referring to times before or after the current
        time, respectively. For complete time and date specification, see
        systemd.time(7). Note that
        --output=short-full prints timestamps that follow precisely this format.
        
-F, --field=¶Print all possible data values the specified field can take in all entries of the journal.
-N, --fields¶Print all field names currently used in all entries of the journal.
--system, --user¶Show messages from system services and the
        kernel (with --system). Show messages from
        service of current user (with --user).  If
        neither is specified, show all messages that the user can see.
        
-M, --machine=¶Show messages from a running, local container. Specify a container name to connect to.
-D DIR, --directory=DIR¶Takes a directory path as argument. If
        specified, journalctl will operate on the specified journal
        directory DIR instead of the
        default runtime and system journal paths.
--file=GLOB¶Takes a file glob as an argument. If
        specified, journalctl will operate on the specified journal
        files matching GLOB instead of the
        default runtime and system journal paths. May be specified
        multiple times, in which case files will be suitably
        interleaved.
--root=ROOT¶Takes a directory path as an argument. If specified, journalctl
        will operate on journal directories and catalog file hierarchy underneath the specified directory
        instead of the root directory (e.g. --update-catalog will create
        , and journal
        files under ROOT/var/lib/systemd/catalog/database or
        ROOT/run/journal/ will be displayed).
        ROOT/var/log/journal/
--image=IMAGE¶Takes a path to a disk image file or block device node. If specified,
        journalctl will operate on the file system in the indicated disk image. This is
        similar to --root= but operates on file systems stored in disk images or block
        devices, thus providing an easy way to extract log data from disk images. The disk image should
        either contain just a file system or a set of file systems within a GPT partition table, following
        the Discoverable Partitions
        Specification. For further information on supported disk images, see
        systemd-nspawn(1)'s
        switch of the same name.
--namespace=NAMESPACE¶Takes a journal namespace identifier string as argument. If not specified the data
        collected by the default namespace is shown. If specified shows the log data of the specified
        namespace instead. If the namespace is specified as "*" data from all namespaces is
        shown, interleaved. If the namespace identifier is prefixed with "+" data from the
        specified namespace and the default namespace is shown, interleaved, but no other. For details about
        journal namespaces see
        systemd-journald.service(8).
--header¶Instead of showing journal contents, show internal header information of the journal fields accessed.
This option is particularly useful when trying to identify out-of-order journal entries, as happens for example when the machine is booted with the wrong system time.
--disk-usage¶Shows the current disk usage of all journal files. This shows the sum of the disk usage of all archived and active journal files.
--vacuum-size=, --vacuum-time=, --vacuum-files=¶Removes the oldest archived journal files until the disk space they use falls below the
        specified size (specified with the usual "K", "M", "G" and
        "T" suffixes), or all archived journal files contain no data older than the specified timespan
        (specified with the usual "s", "m", "h",
        "days", "months", "weeks" and "years"
        suffixes), or no more than the specified number of separate journal files remain. Note that running
        --vacuum-size= has only an indirect effect on the output shown by
        --disk-usage, as the latter includes active journal files, while the vacuuming operation only
        operates on archived journal files. Similarly, --vacuum-files= might not actually reduce the
        number of journal files to below the specified number, as it will not remove active journal
        files.
--vacuum-size=, --vacuum-time= and --vacuum-files=
        may be combined in a single invocation to enforce any combination of a size, a time and a number of files limit
        on the archived journal files. Specifying any of these three parameters as zero is equivalent to not enforcing
        the specific limit, and is thus redundant.
These three switches may also be combined with --rotate into one command. If so, all
        active files are rotated first, and the requested vacuuming operation is executed right after. The rotation has
        the effect that all currently active files are archived (and potentially new, empty journal files opened as
        replacement), and hence the vacuuming operation has the greatest effect as it can take all log data written so
        far into account.
--list-catalog
        [128-bit-ID…]
        ¶List the contents of the message catalog as a table of message IDs, plus their short description strings.
If any 128-bit-IDs are
        specified, only those entries are shown.
--dump-catalog
        [128-bit-ID…]
        ¶Show the contents of the message catalog, with
        entries separated by a line consisting of two dashes and the
        ID (the format is the same as .catalog
        files).
If any 128-bit-IDs are
        specified, only those entries are shown.
--update-catalog¶Update the message catalog index. This command needs to be executed each time new catalog files are installed, removed, or updated to rebuild the binary catalog index.
--setup-keys¶Instead of showing journal contents, generate
        a new key pair for Forward Secure Sealing (FSS). This will
        generate a sealing key and a verification key. The sealing key
        is stored in the journal data directory and shall remain on
        the host. The verification key should be stored
        externally. Refer to the Seal= option in
        journald.conf(5)
        for information on Forward Secure Sealing and for a link to a
        refereed scholarly paper detailing the cryptographic theory it
        is based on.
--force¶When --setup-keys is passed
        and Forward Secure Sealing (FSS) has already been configured,
        recreate FSS keys.
--interval=¶Specifies the change interval for the sealing
        key when generating an FSS key pair with
        --setup-keys. Shorter intervals increase CPU
        consumption but shorten the time range of undetectable journal
        alterations. Defaults to 15min.
--verify¶Check the journal file for internal
        consistency. If the file has been generated with FSS enabled and
        the FSS verification key has been specified with
        --verify-key=, authenticity of the journal file
        is verified.
--verify-key=¶Specifies the FSS verification key to use for
        the --verify operation.
--sync¶Asks the journal daemon to write all yet unwritten journal data to the backing file system and synchronize all journals. This call does not return until the synchronization operation is complete. This command guarantees that any log messages written before its invocation are safely stored on disk at the time it returns.
--flush¶Asks the journal daemon to flush any log data stored in
        /run/log/journal/ into /var/log/journal/, if persistent
        storage is enabled. This call does not return until the operation is complete. Note that this call is
        idempotent: the data is only flushed from /run/log/journal/ into
        /var/log/journal/ once during system runtime (but see
        --relinquish-var below), and this command exits cleanly without executing any
        operation if this has already happened. This command effectively guarantees that all data is flushed
        to /var/log/journal/ at the time it returns.
--relinquish-var¶Asks the journal daemon for the reverse operation to --flush: if
        requested the daemon will write further log data to /run/log/journal/ and stops
        writing to /var/log/journal/. A subsequent call to --flush
        causes the log output to switch back to /var/log/journal/, see
        above.
--smart-relinquish-var¶Similar to --relinquish-var but executes no operation if the root file
        system and /var/lib/journal/ reside on the same mount point. This operation is
        used during system shutdown in order to make the journal daemon stop writing data to
        /var/log/journal/ in case that directory is located on a mount point that needs
        to be unmounted.
--rotate¶Asks the journal daemon to rotate journal files. This call does not return until the rotation
        operation is complete. Journal file rotation has the effect that all currently active journal files are marked
        as archived and renamed, so that they are never written to in future. New (empty) journal files are then
        created in their place. This operation may be combined with --vacuum-size=,
        --vacuum-time= and --vacuum-file= into a single command, see
        above.
-h, --help¶--version¶--no-pager¶Do not pipe output into a pager.
$SYSTEMD_LOG_LEVEL¶The maximum log level of emitted messages (messages with a higher
      log level, i.e. less important ones, will be suppressed). Either one of (in order of decreasing
      importance) emerg, alert, crit,
      err, warning, notice,
      info, debug, or an integer in the range 0…7. See
      syslog(3)
      for more information.
$SYSTEMD_LOG_COLOR¶A boolean. If true, messages written to the tty will be colored according to priority.
This setting is only useful when messages are written directly to the terminal, because journalctl(1) and other tools that display logs will color messages based on the log level on their own.
$SYSTEMD_LOG_TIME¶A boolean. If true, console log messages will be prefixed with a timestamp.
This setting is only useful when messages are written directly to the terminal or a file, because journalctl(1) and other tools that display logs will attach timestamps based on the entry metadata on their own.
$SYSTEMD_LOG_LOCATION¶A boolean. If true, messages will be prefixed with a filename and line number in the source code where the message originates.
Note that the log location is often attached as metadata to journal entries anyway. Including it directly in the message text can nevertheless be convenient when debugging programs.
$SYSTEMD_LOG_TID¶A boolean. If true, messages will be prefixed with the current numerical thread ID (TID).
Note that the this information is attached as metadata to journal entries anyway. Including it directly in the message text can nevertheless be convenient when debugging programs.
$SYSTEMD_LOG_TARGET¶The destination for log messages. One of
      console (log to the attached tty), console-prefixed (log to
      the attached tty but with prefixes encoding the log level and "facility", see syslog(3),
      kmsg (log to the kernel circular log buffer), journal (log to
      the journal), journal-or-kmsg (log to the journal if available, and to kmsg
      otherwise), auto (determine the appropriate log target automatically, the default),
      null (disable log output).
$SYSTEMD_PAGER¶Pager to use when --no-pager is not given; overrides
      $PAGER. If neither $SYSTEMD_PAGER nor $PAGER are set, a
      set of well-known pager implementations are tried in turn, including
      less(1) and
      more(1), until one is found. If
      no pager implementation is discovered no pager is invoked. Setting this environment variable to an empty string
      or the value "cat" is equivalent to passing --no-pager.
$SYSTEMD_LESS¶Override the options passed to less (by default
      "FRSXMK").
Users might want to change two options in particular:
See less(1) for more discussion.
$SYSTEMD_LESSCHARSET¶Override the charset passed to less (by default "utf-8", if
      the invoking terminal is determined to be UTF-8 compatible).
$SYSTEMD_PAGERSECURE¶Takes a boolean argument. When true, the "secure" mode of the pager is enabled; if
      false, disabled. If $SYSTEMD_PAGERSECURE is not set at all, secure mode is enabled
      if the effective UID is not the same as the owner of the login session, see
      geteuid(2)
      and sd_pid_get_owner_uid(3).
      In secure mode, LESSSECURE=1 will be set when invoking the pager, and the pager shall
      disable commands that open or create new files or start new subprocesses. When
      $SYSTEMD_PAGERSECURE is not set at all, pagers which are not known to implement
      secure mode will not be used. (Currently only
      less(1)
      implements secure mode.)
Note: when commands are invoked with elevated privileges, for example under sudo(8) or
      pkexec(1), care
      must be taken to ensure that unintended interactive features are not enabled. "Secure" mode for the
      pager may be enabled automatically as describe above. Setting SYSTEMD_PAGERSECURE=0
      or not removing it from the inherited environment allows the user to invoke arbitrary commands. Note
      that if the $SYSTEMD_PAGER or $PAGER variables are to be
      honoured, $SYSTEMD_PAGERSECURE must be set too. It might be reasonable to completely
      disable the pager using --no-pager instead.
$SYSTEMD_COLORS¶Takes a boolean argument. When true, systemd and related utilities
      will use colors in their output, otherwise the output will be monochrome. Additionally, the variable can
      take one of the following special values: "16", "256" to restrict the use
      of colors to the base 16 or 256 ANSI colors, respectively. This can be specified to override the automatic
      decision based on $TERM and what the console is connected to.
$SYSTEMD_URLIFY¶The value must be a boolean. Controls whether clickable links should be generated in
      the output for terminal emulators supporting this. This can be specified to override the decision that
      systemd makes based on $TERM and other conditions.
Without arguments, all collected logs are shown unfiltered:
journalctl
With one match specified, all entries with a field matching the expression are shown:
journalctl _SYSTEMD_UNIT=avahi-daemon.service journalctl _SYSTEMD_CGROUP=/user.slice/user-42.slice/session-c1.scope
If two different fields are matched, only entries matching both expressions at the same time are shown:
journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=28097
If two matches refer to the same field, all entries matching either expression are shown:
journalctl _SYSTEMD_UNIT=avahi-daemon.service _SYSTEMD_UNIT=dbus.service
If the separator "+" is used, two
    expressions may be combined in a logical OR. The following will
    show all messages from the Avahi service process with the PID
    28097 plus all messages from the D-Bus service (from any of its
    processes):
journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=28097 + _SYSTEMD_UNIT=dbus.service
To show all fields emitted by a unit and about
    the unit, option -u/--unit= should be used.
    journalctl -u name
    expands to a complex filter similar to
    
_SYSTEMD_UNIT=name.service + UNIT=name.service _PID=1 + OBJECT_SYSTEMD_UNIT=name.service _UID=0 + COREDUMP_UNIT=name.service _UID=0 MESSAGE_ID=fc2e22bc6ee647b6b90729ab34a250b1
(see systemd.journal-fields(7) for an explanation of those patterns).
Show all logs generated by the D-Bus executable:
journalctl /usr/bin/dbus-daemon
Show all kernel logs from previous boot:
journalctl -k -b -1
Show a live log display from a system service
    apache.service:
journalctl -f -u apache